The 30 trumpeters will be 6 times greater intensity than the 5 trumpeters. The ratio in decibels all "drop out" leaving log(6) = 0.78 (about).
30 trumpeters are louder than 6, so to get the new decibels it is 75 / 0.78 = 96 decibels.

Logic check - 6 x intensity is < 10 x, so we expect the difference in decibels to be < 2x.